An X-ray phosphor is a compound which emits actinic radiation when excited by X-radiation. In medical imaging, the phosphor is present in a so-called X-ray intensifying screen which is exposed to X-rays and the emitted light exposes a film for eventual processing and viewing of an image. Such phosphors and screens made therefrom are well known to those of ordinary skill in the art.
The efficiency of phosphors, and X-ray phosphors in particular, can be measured in a number of ways. Perhaps the most basic method of measuring the efficiency of an X-ray phosphor is to expose the phosphor to X-ray photons and, with instrumentation, measure the number of.sup.* FNT *X-ray photons absorbed by the phosphor and the number of light photons emitted by the phosphor.
This technique would measure the inherent X-ray to light conversion efficiency of the phosphor material, which is generally believed to be a physical constant.
A more common method of measuring a phosphor's efficiency is by measuring the speed (i.e., brightness) of an X-ray intensifying screen containing such phosphor. In this method, the speed of the screen is directly related to the phosphor coating weight and directly related to the size of the phosphor particles. In other words, for a given phosphor, the speed of an X-ray intensifying screen can be reduced by lowering the phosphor coating weight or by decreasing the size of the phosphor particles. An additive effect is typically seen in combining these two variables. The methods of preparing X-ray phosphors are widely known with the common steps being (1) mixing of phosphor precursor materials; (2) grinding to increase the surface area; and (3) firing at a high temperature to allow the precursors to react and form the phosphor. It is also common to have a flux material present during one or more of these steps to aid in the firing. After firing, the mixture is washed and dried and the phosphor is recovered.
Typical phosphor precursor materials are oxides or other salts of the rare earths, oxides or other salts of transition metal elements or combinations thereof. Most of these compounds are very abrasive and difficult to grind. The quality of the resulting phosphor and the efficiency of the process are largely dependent upon the grinding step. The grinding step is also a common source of contamination of the phosphor, particularly due to the abrasion of the internal parts of the grinding apparatus. Therefore, the grinding step has received a substantial degree of attention in the ongoing search for increased phosphor quality and process efficiency.
It is customary in the art to utilize a ball mill containing particulate media to grind the precursor materials and optionally the flux. Ball mills are, however, disadvantageous in that particle size distribution is not easily controlled and the process capacity is controlled by the optimal batch size for the ball mill. Additionally, this method of grinding often is accomplished with an inert organic solvent, such as chloroflourocarbons, which are environmentally disadvantageous.
Fluid grinding mills, as exemplified in U.S. Pat. Nos. 3,229,918 and 3,726,484, have heretofore been employed for grinding of pigments and the like. Briefly, fluid grinding mills comprise shallow cylindrical grinding chambers having a plurality of circumferentially spaced fluid inlet ports, a feed port and an exit port. In operation, the material to be ground is aspirated into the grinding chamber by a stream of transport fluid, such as air or an inert gas. Inside the chamber, the fluid flow minimizes particle collisions with the chamber walls and maximizes particle--particle collisions, which is the primary means of grinding. The grinding action continues until the particles as of such size as to be aspirated out of the chamber via the exit port.
The primary advantages of fluid grinding mills are the elimination of moving parts, the ability to use a continuous process instead of a batch process resulting in improved process efficiency and capacity, and the elimination of the use of organic solvents in the grinding step. The grinding mill of U.S. Pat. No. 3,229,918 also offers the advantage of removable liners for rapid revitalization of the grinding surfaces.